Question

我正在努力弄清楚特定查询的Linq-to-SQL语法是什么。我可以在SQL中轻松完成这项工作，但我无法在Linq中获得正确的语法。

我在两个数据库表中有父记录和子记录，由外键链接。我希望我的结果根据这些规则返回行：

无论有多少孩子存在，每个父母都会返回 1行。
如果孩子不存在，则返回null / zero值。
从具有空条件的子项返回相关数据。如果存在多个具有空条件的多个，则仅返回第一个一个。
返回具有非空条件的子记录数。

我已经在.NET Fiddle中玩了一段时间，但我无法做到这一点。这是我到目前为止（忽略随机描述！）：

    IEnumerable<Parent> parents = new Parent[] { 
        new Parent { ID = 1, Description = "Apple" },
        new Parent { ID = 2, Description = "Orange" },
        new Parent { ID = 3, Description = "Pear" },
        new Parent { ID = 4, Description = "Banana" } };

    IEnumerable<Child> children = new Child[] {
        new Child { ID = 1, ParentID = 2, Description = "Mercury", Condition = null },
        new Child { ID = 2, ParentID = 3, Description = "Venus", Condition = null },
        new Child { ID = 3, ParentID = 3, Description = "Earth", Condition = null },
        new Child { ID = 4, ParentID = 4, Description = "Mars", Condition = null },
        new Child { ID = 5, ParentID = 4, Description = "Saturn", Condition = "> 5" } };

    /// What goes here...?
    var query = from p in parents
                join c in children on p.ID equals c.ParentID into jc                    
                from subchildren in jc.DefaultIfEmpty()
                select new Item { 
                    ParentID = p.ID, 
                    Description = p.Description, 
                    PrimaryChildID = subchildren == null ? 0 : subchildren.ID, 
                    SubDescription = subchildren == null ? null : subchildren.Description,
                    ConditionalCount = 0};

    foreach (var item in query) 
        Console.WriteLine("{0} {1} {2} {3} {4}",
            item.ParentID, 
            item.PrimaryChildID,                              
            item.Description,                
            item.SubDescription, 
            item.ConditionalCount);

我得到的输出是：

1 0 Apple  0
2 1 Orange Mercury 0
3 2 Pear Venus 0
3 3 Pear Earth 0
4 4 Banana Mars 0
4 5 Banana Saturn 0

但我想要这个：

1 0 Apple  0
2 1 Orange Mercury 0
3 2 Pear Venus 0
4 4 Banana Mars 1

任何人都可以帮我解决此查询的正确语法吗？

Answer 1

在您的情况下，您不需要left join，而是需要group join。

根据MSDN： -

The group join is useful for producing hierarchical data structures. 
It pairs each element from the first collection with a set of correlated elements
from the second collection.

这样做： -

var query = from p in parents
       join c in children
       on p.ID equals c.ParentID into g
       let firstNullElement = g.FirstOrDefault(x => x.Condition == null)
       select new
      {
         ParentID = p.ID,
         PrimaryChildID = firstNullElement != null ? firstNullElement.ID : 0,
         Description = p.Description,
         SubDescription = firstNullElement!= null ? firstNullElement.Description
                                                  : String.Empty,
         ConditionalCount = g.Count(x => x.Condition != null)
      };

为了正确解释，以下是在我们使用select new { }投影实际所需数据之前生成的内容，（证明Group Join的定义是合理的）： -

ParentId                    g
----------------------------------------------
1                          null

2          ID = 1, ParentID = 2, Description = "Mercury", Condition = null

3          ID = 2, ParentID = 3, Description = "Venus", Condition = null
           ID = 3, ParentID = 3, Description = "Earth", Condition = null

4          ID = 4, ParentID = 4, Description = "Mars", Condition = null
           ID = 5, ParentID = 4, Description = "Saturn", Condition = "> 5"

现在，由于g持有IEnumerable个子元素，我们可以应用过滤器，项目数据，计数或做任何我们想做的事情，就像我们在使用select的最终语句中所做的那样。而且，正如我们所看到的，没有任何数据来自不同的子元素。

以下是完整的Working Fiddle。

Answer 2

这应该做的事情

    var query = (from p in parents
                     select new
                     {
                         ParentID = p.ID,
                         Description = p.Description,

                         PrimaryChildID = children.Where(c => c.ParentID == p.ID && c.Condition == null).Count() == 0 ? 0 : children.OrderBy(c=>c.ID).FirstOrDefault(c => c.ParentID == p.ID && c.Condition == null).ID,
                         SubDescription = children.Where(c => c.ParentID == p.ID && c.Condition == null).Count() == 0 ? null : children.OrderBy(c => c.ID).FirstOrDefault(c => c.ParentID == p.ID && c.Condition == null).Description,
                         ConditionalCount = children.Where(c => c.ParentID == p.ID && c.Condition != null).Count()
                     }).ToList();

Answer 3

这是我的查询变体：

        var query = from p in parents
                    join c in children on p.ID equals c.ParentID into jc
                    from subchildren in jc.DefaultIfEmpty()
                    select new
                    {
                        Parent = p,
                        Subchildren = subchildren
                    } into itemData
                    group itemData by itemData.Parent into g
                    select new Item
                    {
                        ParentID = g.Key.ID,
                        Description = g.Key.Description,
                        PrimaryChildID = g.Select(_ => _.Subchildren == null ? 0 : _.Subchildren.ID).FirstOrDefault(),
                        SubDescription = g.Select(_ => _.Subchildren == null ? null : _.Subchildren.Description).FirstOrDefault(),
                        ConditionalCount = 0
                    };

复杂的Linq到SQL查询

3 个答案: